The subject matter of the present invention is the recovery of nickel values from a low-grade nickel complex ore. The recovery comprises the leaching, under elevated temperatures, of the ore with an aqueous ammoniacal solution for relatively short, preferably about an hour or less, periods of time.
Commercially significant nickel deposits are of two main types: (i) sulphides which are primary nickel ores or rock materials and (ii) laterites and garnierites which are secondary nickel ores or rock materials. The nickel bearing laterites and garnierites result from the deterioration of primary rock materials through weathering, erosion and related chemical and physical processes during which the nickel values are greatly concentrated compared with the primary rock materials and are deposited in layers of altered residual rock materials. These nickel bearing laterites and garnierites normally contain from about 1.5 to about 3 weight percent nickel.
Primary rock materials with a low nickel content, typically below 0.5 weight percent, are found in various Precambrian and Cordillera regions in Alaska, Asia, Australia, Canada, Northern Europe and in various tropical and subtropical regions. In Canada the low-grade primary nickel ores are characteristically found in the form of peridotite or other ultramafic rock formations. These formations represent huge reserves of nickel. However, due to the low nickel values of these ores it is difficult to extract the nickel on a commercially viable basis. Several techniques have been used in attempting to extract the nickel from said ores commercially. These methods include various flotation, magnetic separation, and roast-leach processes. However, these techniques have not proved to be entirely satisfactory, either because of rather high costs or because of rather small amounts of nickel extracted.
The present invention resides in the discovery that in a high temperature leach of the low-grade nickel ore the maximum solubilization of nickel in the leach solution occurred within a matter of minutes after leaching began and thereafter decreased dramatically with increased leaching time. This is in sharp contrast to known leaching techniques wherein the amount of nickel extracted into the leach solution increases with the increase in length of time that leaching is carried out. Past practice has been to leach the ore for at least 24 hours, and often for periods of up to several days and even weeks.